The present invention relates to a thermal management device for managing the dissipation of heat in, for example, electronic equipment and a method of making such a device. In particular, the invention relates to a thermal management device that has electrical feed through capability and can act as a direct interface to active elements.
Electronic and electrical devices are the sources of both power and heat. As is well known, in order to provide reliable operation of such devices, it is necessary to maintain stable operating conditions and temperatures. Hence, efficient methods for heat management and dissipation are essential. Typically this is done by providing thermal management devices that are arranged adjacent and in contact with the electronic device or circuit board. Heat generated in the circuit is transferred to and dissipated in the thermal management device. For optimum efficiency, it is desirable that thermal management structures have the highest possible thermal conductivity, efficient external connectivity and appropriate mechanical strength.
To achieve these objectives in thermally demanding applications, some known devices encapsulate high thermal conductivity materials into composite structures. However, these devices often achieve only limited performance, with significant conductivity losses, typically 40%, and increases in mass and bulk Examples of such structures are described in EP 0,147,014, EP 0,428,458, U.S. Pat. No. 5,296,310, U.S. Pat. No. 4,791,248 and EP 0,231.823. The best thermal management systems available at present have conductivities that typically do not exceed 1,000 W/mK.
Current technologies do not provide thermal management that is sufficient in many applications whilst at the same time providing efficient electrical interconnection between layers or sides of circuit boards. A further problem is that the mass and volume of known thermal management systems are relatively large. This affects the overall size of electronic systems in which such devices are incorporated. In this day and age when the general drive of the electronics industry is towards miniaturisation, this is highly disadvantageous.
Thermal management systems are often used as substrates for supports for hybrid electronic circuits. In one known arrangement, beryllia is used as a heat sink. This has a thermal conductivity of around 280 W/mK at room temperature. On top of this is a layer of dielectric on which gold contacts are subsequently formed, thereby to enable connection to other electrical circuits. A disadvantage of this arrangement is that beryllia is a hazardous material, in fact it is carcinogenic, and is generally difficult to process. In addition, the dielectric tends to be thick thereby making the overall structure bulky. Furthermore, partly because of the use of gold as a contact material, the overall structure is expensive to manufacture.
An object of the present invention is to provide a thermal management system that has a high thermal conductivity but a low mass and volume.
According to a first aspect of the present invention there is provided a thermal management device comprising anisotropic carbon encapsulated in an encapsulating material that is applied directly to the carbon and is able to improve the rigidity of the carbon, preferably wherein the encapsulating material is polyimide or epoxy resin or acrylic or polyurethane or polyester or any other suitable polymer.
Preferably, the anisotropic carbon has mosaic or full ordering.
Preferably, the anisotropic carbon is thermalised pyrolytic graphite that has mosaic or full ordering. The thermalised pyrolytic graphite may have an in plane thermal conductivity of 1550-1850 W/mK at around room temperature. Typically, the thermalised pyrolitic graphite has a low value of tensile strength in the orthogonal direction.
The anisotropic carbon may alternatively be pyrolytic graphite. The pyrolytic graphite may be in an xe2x80x9cas depositedxe2x80x9d or partially ordered form. The conductivity of the pyrolytic graphite may be in the range of 300-420 W/mK in one plane. The tensile strength of the plate may be 1.5 Ksi in the orthogonal plane.
Preferably, the anisotropic carbon is a plate. Preferably the carbon plate has a thickness in the range 100-500 xcexcm. The carbon plate may have a thickness in the range of 200-250 xcexcm or 250-300 xcexcm or 300-350 xcexcm or 350-400 xcexcm or 400-450 xcexcm or 450-500 xcexcm.
Preferably the material encapsulating the carbon has a low thermal expansion coefficient and high degradation temperature, such as a polyimide, for example PI 2734 provided by DuPont (trade mark), where the thermal expansion coefficient is around 13 ppm/C and the degradation temperature is around 500 C.
The coating layer may have a thickness in the range from a few microns to many tens of microns. Multiple layers of coating may be formed on the carbon in order to build up a desired thickness.
A matrix of fine holes, preferably 200 xcexcm diameter. may be formed through the carbon plate, prior to encapsulation. These holes are filled during encapsulation of the plate. An advantage of this is that it reduces the possibility of internal delamination.
According to a second aspect. of the present invention. there is provided an electrical system comprising a thermal management device in which the first aspect of the invention is embodied, on a surface of which electrical contacts and/or devices are provided.
The devices may be deposited directly on the surface or may be glued using, for example, a thin layer of liquid glue. Preferably, the devices are encapsulated in polyimide or epoxy resin or acrylic or polyurethane or polyester or any other suitable polymer.
Preferably, a plurality of layers of electrical components are provided, each spaced apart by layers of polyimide. Typically, the electrical contacts are made of thin film metal, for example aluminium.
According to a third aspect of the present invention there is provided a method of fabricating a thermal management device comprising:
applying a coat of encapsulating material, preferably polyimide or epoxy
resin or acrylic or polyurethane or polyester or any other suitable
polymer directly to a clean carbon surface, the encapsulating material
being such as to improve the rigidity of the carbon; and
repeating the foregoing steps until the carbon is encapsulated.
The method may additionally involve curing the encapsulating material.
Preferably, the step of applying involves brushing, rolling, dipping, spraying, spinning, stamping or screen-printing. Preferably, for polyimide, which consists of a single-component, the step of applying the coating involves brushing the polyimide or applying it using a roller. For solid phase application a cast can be used. This requires a pre-polymerised foil of the encapsulating material to be applied directly on to the clean surface. This can be useful when simple thermal management devices are required with no internal holes. Preferably, the carbon and cast are compressed within a vacuum and at high temperature.
Preferably, the step of applying involves applying multiple layers of encapsulating material, such as polyimide or epoxy resin or acrylic or polyurethane or polyester or any other suitable polymer, until a desired thickness is reached.
Preferably, the method includes cleaning a surface of the carbon thereby to produce said clean carbon surface.
Preferably, the step of cleaning involves using pumice powder under water to remove loose materials, followed by drying. Preferably, the step of drying involves drying the carbon by baking the carbon surface to remove moisture, for example, at 100 C for one hour.
Preferably the step of cleaning includes degreasing the carbon by, for example, rinsing it with acetone.
When polyimide is used, it is preferable that the step of curing involves heating the carbon to 150 C for, for example, 1 hour and subsequently temperature cycling the board to 150 C for 30 minutes, 250 C for 30 minutes and finally 300 C for 30 minutes.
In the case of epoxy, this can consist of a single component or else be a double component mixture. For the single component type then a two stage gluing can be carried out by firstly drying the glue to remove the solvent at a given temperature, (typically around 120 C.) and form a solid phase, and then heating it at a higher temperature typically around 180xc2x0 to complete the polymerisation. In the case of double component epoxy, the initial mixing of the components causes the polymerisation process to begin, and the process may then need anything between minutes and several hours for the process to be completed, depending upon the particular epoxy.
Preferably the method further comprises drilling the carbon with at least one hole prior to application of the encapsulating material. The at least one hole may be completely infilled with encapsulating material. The holes may be infilled with encapsulating material that is mixed with glass fibre spheres, each sphere typically having a diameter of 30 xcexcm. This process may be carried out before the pure polyimide coating is used to encapsulate the surface of the plate, and can improve the uniformity of coating thickness across the surface of the plate by preventing the possibility of thinning occurring around the edges of the initial holes in the plates. In either case, once the encapsulation process is completed, the said at least one hole is re-drilled. thereby to provide a through hole that is electrically insulated from the carbon core.
Preferably, a layer of a conducting material is applied to the at least one hole to produce electrical connections, thereby to enable electrical connections through the carbon. Preferably, the conducting material is a metal, for example thin film aluminium. Alternatively, the edges that define the at least one hole may be coated with the encapsulating material in such a way as to maintain a passage through the carbon, thereby to avoid having to conduct the step of drilling through encapsulating material.
The method may further involve forming a matrix of fine holes through the carbon. These holes are of course infilled when the plate is filly encapsulated.
According to a fourth aspect of the present invention, there is provided a method of fabricating an electrical component comprising the method of the third aspect of the present invention and additionally the steps of forming electrical contacts on at least one surface of the carbon and/or depositing electrical devices thereon.
The step of depositing may involve fabricating the devices directly on the surface or forming the devices or a thin film multi-layer circuit containing the devices separately from the carbon surface and fixing them to that surface. Preferably, the step of fixing involves applying glue to the devices or the circuit or the carbon surface and pressing the devices or circuit and the surface together at room temperature and at low vacuum.
Preferably, the electrical contacts are applied using thin film processing techniques using, for example, aluminium.
Various devices and methods in which the present invention is embodied will now be described by way of example only and with reference to the following drawings, of which: